An inkjet printing system typically includes one or more printheads and their corresponding ink supplies. Each printhead includes an ink inlet that is connected to its ink supply and an array of drop ejectors, each ejector consisting of an ink pressurization chamber, an ejecting actuator and an orifice through which droplets of ink are ejected. The ejecting actuator may be one of various types, including a heater that vaporizes some of the ink in the pressurization chamber in order to propel a droplet out of the orifice, or a piezoelectric device which changes the wall geometry of the chamber in order to generate a pressure wave that ejects a droplet. The droplets are typically directed toward paper or other recording medium, i.e., print medium, (also sometimes generically referred to as paper herein) in order to produce an image according to image data that is converted into electronic firing pulses for the drop ejectors as the print medium is moved relative to the printhead.
Motion of the print medium relative to the printhead may consist of keeping the printhead stationary and advancing the print medium past the printhead while the drops are ejected. This architecture is appropriate if the nozzle array on the printhead can address the entire region of interest across the width of the print medium. Such printheads are sometimes called pagewidth printheads.
A second type of printer architecture is the carriage printer, where the printhead nozzle array is somewhat smaller than the extent of the region of interest for printing on the print medium and the printhead is mounted on a carriage. In a carriage printer, the print medium is advanced a given distance along a print medium advance direction and then stopped. While the print medium is stopped, the printhead carriage is moved in a direction that is substantially perpendicular to the print medium advance direction as the drops are ejected from the nozzles. After the carriage has printed a swath of the image while traversing the print medium, the print medium is advanced; the carriage direction of motion is reversed; and the image is formed swath by swath.
Inkjet ink includes a variety of volatile and nonvolatile components including pigments or dyes, humectants, image durability enhancers, and carriers or solvents. A key consideration in ink formulation is the ability to produce high quality images on the print medium. During periods when ink is not being ejected from an ejector, the ink viscosity at the nozzle can change. For example, the volatile components of the ink can evaporate through the nozzle. Such changes can make the drop ejection process nonuniform, so that the image quality can be degraded. In addition, dust, dried ink or other particulates can partially block a nozzle or make the wettability of the nozzle face around the nozzle nonuniform so that ejected drops can be misdirected from their intended flight paths.
In order to maintain the drop ejecting quality of the printhead so that high quality images are produced even after periods where one or more nozzles have been inactive, a variety of maintenance actions has been developed and is well known in the art. These maintenance actions can include capping the printhead nozzle face region during periods of nonprinting, wiping the nozzle face, periodically spitting drops from the nozzles into the cap or other reservoir that is outside the printing region, priming the nozzles by applying a suction pressure at the nozzle face, and etc.
Although most maintenance operations are performed to maintain drop ejecting quality in a direct manner as described above, some maintenance operations are performed in an indirect manner. An example of such an indirect maintenance operation is disclosed in U.S. Pat. No. 5,404,158, where the printhead can eject ink into the cap for the purpose of increasing the humidity within the cap. Such a maintenance operation is herein termed an indirect operation because it is maintaining proper conditions within the cap so that the cap will be able to provide suitable surroundings for the printhead.
Maintenance operations use ink that would otherwise be available for printing. What is needed is a way to control maintenance operations, and more specifically a way to control indirect maintenance operations, such that ink is used more efficiently. More efficient ink usage makes it possible for the user to change ink supplies less frequently, which results in saving the user both effort and money, and also putting less waste into the environment.